1. Field of the Invention
This invention relates generally to sensing amplifiers and, more particularly, is directed to a sensing amplifier for a random access memory (RAM) that is suitably used to amplify a signal read out from a memory.
2. Description of the Prior Art
In the prior art, a sensing amplifier is used to amplify a signal having quite a low level read out from an internal storage or external storage (memory) of an electronic computer such that the level of such a signal read out reaches the voltage level to be processable by a logic circuit.
An example of a prior art sensing amplifier will be described with reference to FIG. 1. FIG. 1 is a schematic diagram showing a construction of such a prior art sensing amplifier.
Referring to FIG. 1, a memory cell 10 is provided, which is a component of a metal oxide semiconductor (MOS) memory matrix, not shown. The memory cell 10 includes a flip-flop circuit formed of load resistors 11 and 12, and metal oxide semiconductor field effect transistors (MOSFET's) 13 and 14. This memory cell 10 is of a static type memory which stores information each time the MOSFET's 13 and 14 are turned on or off. When the memory cell 10 is put in the reading mode, if an X address (word line) 21 is selected by an X decoder, not shown, MOSFET's 15 and 16 connected to this word line 21 are turned on, and the information stored in the memory cell 10 is transferred to Y addresses (bit lines) 22a and 22b. In this case, since the memory cells on the same word line 21 are all activated, a selection signal for predetermined Y addresses (bit lines) 22a and 22b are supplied through a selection line 23 to MOSFET's 24a and 24b from a Y decoder, not shown. Then, the MOSFET's 24a and 24b are turned on, supplying the information of the predetermined memory cell 10 through the bit lines 22a and 22b to the sensing amplifier.
A first stage amplifying circuit 30 of the sensing amplifier is formed as a current mirror type differential amplifying circuit. To be more concrete, the first stage amplifying circuit 30 comprises N-channel differential input MOSFET's 31 and 32 whose gates are respectively connected to the bit lines 22a and 22b. The sources of both the MOSFET's 31 and 32 are connected together to a drain of a third N-channel MOSFET 33 that serves as a constant current source. The source of the MOSFET 33 is grounded and the gate thereof is connected to a voltage source terminal TP and thereby the MOSFET 33 is turned on. The drains of both the N-channel MOSFET's 31 and 32 are connected respectively to drains of a pair of P-channel MOSFET's 34 and 35, each of which is used as an active load. The gate and the drain of the P-channel MOSFET 34 are directly coupled together to serve as a diode, while the gate of the other P-channel MOSFET 35 is connected to the gate of the MOSFET 34. Then, the sources of both the P-channel MOSFET's 34 and 35 are connected to the power source terminal TP. Thus, the current mirror circuit is constructed.
In the above-mentioned differential amplifying circuit 30, potentials or voltages having voltage levels corresponding to the on or off state of both the input differential MOSFET's 13 and 14 in the memory cell 10 and generated on both the bit lines 22a and 22b are supplied to the gates of both the input differential MOSFET's 31 and 32 as input signals thereof. A difference signal between these input signals is amplified and from a connection point A between the MOSFET's 32 and 35, an unbalanced output signal of the differential amplifying circuit 30 is supplied to a driving stage, inverting amplifying circuit 36. The output signal from this inverting, amplifying circuit 36 is supplied to an output stage, buffer amplifying circuit 37. The output signal from the output amplifying circuit 37 has already reached a predetermined voltage level, and it is delivered through an output terminal 38 to a logic circuit, not shown.
A sensing amplifier used for a memory capable of operating at high speed and producing multiple-outputs, the output amplifying circuit 37 has a large driving capacity. Thus, when a load having a large capacity is connected to the output terminal 38, a transient current having a large amplitude flows through the output amplifying circuit 37 to the voltage source and the ground, generating high frequency noises. The noises are supplied through the voltage source and the ground to the differential amplifying circuit 30 in the same phase.
Of course, as is well known in the art, the differential amplifying circuit amplifies a difference signal between two input signals and it does not amplify the input voltages having the same phase. However, in the conventional current mirror type differential amplifying circuit 30 shown in FIG. 1, the MOSFET 34 forming a part of the current mirror circuit is connected so as to operate as a diode and is low in impedance, while the other MOSFET 35 is high in impedance so that the loads to both the input differential FET's 31 and 32 are made asymmetric. As a result, when the high frequency response characteristic of the constant current source FET 33 is not sufficient, the above-mentioned noise components having the same phase can not be cancelled out within the differential amplifying circuit 30 and appear at the junction a between the MOSFET's 32 and 35, preventing a signal from being read out correctly from the memory.